There are many instances where information needs to be transmitted quickly and cheaply. Amplitude modulation (or AM) radio transmission can be easily and inexpensively accomplished and, despite some deficiencies, is very attractive for many applications. In particular, new types of highly-specialized, direct radio advertising can be achieved using AM radios, such as for example, AM radios transmitting in the frequency band 510-1705 kilohertz.
One relatively new application for AM radios is in the sale of real estate. Recently, real estate of various types, but particularly residential homes, has been equipped with a radio transmitter which broadcasts a pre-recorded message describing the real estate and its features. A prospective buyer may then drive to the property and tune his or her automobile radio to the broadcast frequency of the radio transmitter and listen to the pre-recorded message. The system can operate 24 hours a day, seven days a week until the property is sold. Thus, prospective buyers may gather information at any time, not just those times when the property is open for inspection or when a real estate agent is available to show the property. Furthermore, the system has the ability to reach casual buyers who may not presently be interested in purchasing real estate, but who, if attracted to a property they pass by, may listen to a prerecorded message in their automobile. This initial exposure may lead to a purchase in some cases.
While direct radio marketing broadcasts have certain advantages, such broadcasts must meet Federal Communications Commission (FCC) restrictions related to signal frequency and strength. More specifically, the FCC has created specific regulations directed to the operation of unlicensed radio transmitters operating in the frequency band 510-1705 kilohertz. The FCC requires that the total input power supplied to the final radio frequency stage not exceed 100 milliwatts. Power supplied to a filament and/or a heater is not required to be included in the total input power calculation. In addition, the total length of the transmission line, antenna and ground lead (if used) should not exceed a total length of three meters or 9.84 feet.
Another FCC requirement is that direct marketing broadcasts not interfere with other radio signals such as commercial radio stations. Another consideration to be made when designing a transmitter for real estate sales is that several AM radios might be used in the same geographical area if multiple properties in that area are for sale at the same time. The proximity of many radio signals may cause interference, particularly if the radios broadcast on only one frequency. Thus, a direct marketing radio transmitter must be designed so that the frequency of its broadcast signal can be selected from a range of frequencies depending on the specific placement to be made.